The present invention relates generally to scanning movable elements, and more specifically to techniques for performing step scanning in an interferometer used in a Fourier transform spectrometer.
A Fourier transform spectrometer typically includes an interferometer into which are directed an infrared beam to be analyzed and a monochromatic beam that provides a position reference. The interferometer has a fixed mirror and a coil driven movable mirror. In rapid scanning, the movable mirror is driven at a nominally constant velocity over a portion of its travel; in step scanning, the movable mirror is moved intermittently. Each of the input beams is split at a beam splitter with one portion traveling a path that causes it to reflect from the fixed mirror and another portion traveling a path that causes it to reflect from the movable mirror. The portions of each beam recombine at the beam splitter, and the recombined beams are directed to appropriate detectors.
The optical interference between the two beam portions causes the intensity of the monochromatic beam and each frequency component of the infrared beam to vary as a function of the component's optical frequency and the mirror position. The detector output represents the superposition of these components and, when sampled at regular distance intervals, provides an interferogram whose Fourier transform yields the desired spectrum.
In a rapid scan interferometer, when the mirror is moved at a constant speed, the monochromatic beam provides a nominally sinusoidal reference signal whose zero crossings occur each time the moving mirror travels an additional one quarter of the reference wavelength (i.e., for each half wavelength change of retardation). The data acquisition electronics are triggered on these zero crossings to provide regularly sampled values for the interferogram. With the appropriate choice of mirror velocity, the output signal can be made to fall within a convenient range of modulation frequencies, as for example in the audio range. It is known practice to use a servo to lock the monochromatic signal to a fixed clock in order to maintain the mirror speed constant.
It is not always necessary to sample at every zero crossing. The usual requirement (Nyquist's theorem) is that the sampling occur at twice the maximum frequency in the spectral range of interest. For longer wavelengths, it is common practice to sample at every nth zero crossing where n is a small integer. It may also be desired on some occasions to undersample, in which case the sampling would not be at every zero crossing. Thus the reference interval between reference positions may be the distance between zero crossings, or an integral multiple thereof.
In a step scan interferometer, the movable mirror is moved from one reference point to the next and then stopped, at which point an intensity measurement is made. The sequence is then repeated until the desired interferogram has been acquired. The prior art teaches various techniques for accomplishing this under servo control. One approach uses quadrature detection (two sinusoids at 90.degree. relative phase) to provide position and direction information, and changes the reference to allow stepping from one zero crossing to another (possibly a zero crossing of the other sinusoid). The use of quadrature detection in conjunction with a single sideband technique to obtain a sinusoid at a multiple of the reference frequency allows stepping to any of many positions between zero crossings. Another approach uses a dither and detects the fundamental frequency and the second harmonic with separate lock-in amplifiers. By switching between which amplifier provides the error signal, it is possible to step from zero crossing to zero crossing. A single step typically takes on the order of 100 ms.
Phase modulation is a technique wherein a sinusoidal signal is applied to the moving mirror to dither the position around each desired retardation. This is typically by an amount corresponding to .+-.90.degree. of phase shift of the shortest wavelength in the spectral range of interest (103.degree. is optimum). The infrared detector signal is passed through a demodulator such as a lock-in amplifier to detect the signal level at the dither frequency. While the shortest wavelength is modulated by almost 100%, the longer wavelengths are modulated to a lesser degree. The output of the lock-in amplifier at a given retardation value provides a measure of the derivative of the interferometer detector signal at that retardation. The technique has applicability in very slow scanning speeds, where it eliminates the need for the detector channel to operate at a very low frequency.